1. Field of the Invention
The present invention relates to a two-wavelength diffraction element in which diffraction efficiency can be adjusted arbitrarily, a method of designing a diffraction grating of the diffraction element, an optical head device for recording, reproducing or erasing information on an optical information medium such as an optical disc by using the diffraction element and an optical information apparatus including the optical head device, as well as a computer, an optical information medium player, a car navigation system, an optical information medium recorder and an optical disc server each of which includes the optical information apparatus.
2. Description of the Prior Art
At present, various kinds of recording mediums are available for recording and storing digital audio, images and moving pictures as well as document files and data files produced by computers or the like. An optical disc is used as one of the recording mediums. Especially, digital versatile disks (DVDs) which have higher density and larger capacity than conventional compact discs (CDs) and are coming into wide use also in the field of recorders in place of video tape recorders (VTRs) used predominantly currently. Furthermore, the study of a next-generation optical disc having a higher recording density is being conducted in a number of firms and is expected to appear on the market in the near future.
In order to raise recording density of the optical disc, it is considered that a numerical aperture (NA) of a beam incident upon its information recording face is increased. However, if its optical axis tilts at this time, such a problem as an increase in the amount of aberration arises. In order to solve this problem, it is effective to reduce a thickness of a protective layer or a substrate thickness in the optical disc. In this specification, the “substrate thickness” indicates a thickness from an incident face of the beam to the information recording face in the optical disc.
Referring to the history of the optical disc, the first-generation optical disc is the CD in which an infrared beam having a wavelength of 780 to 820 nm is used as a light source, an objective lens has a numerical aperture of 0.45 and the thickness of the substrate is 1.2 mm. The second-generation optical disc is the DVD in which a red beam having a wavelength of 630 to 680 nm is used as a light source, an objective lens has a numerical aperture of 0.6 and the thickness of the substrate is 0.6 mm. Meanwhile, the third-generation optical disc under development currently is an ultrahigh-density optical disc in which a blue beam having a wavelength of 380 to 420 nm is used as a light source, an objective lens has a numerical aperture of 0.85 and the thickness of the substrate is 0.1 mm.
As is seen from the above, the thickness of the substrate becomes thinner for raising recording density. A single optical information apparatus is expected to be capable of recording and reproducing optical discs having different substrate thicknesses and different recording densities in view of its economical aspect and its occupied space. To this end, it is necessary to provide an optical head device including a condensing optical system which is capable of condensing a beam up to its diffraction limit on the optical discs having the different substrate thicknesses.
Meanwhile, tracking control and focusing control are typically necessary for recording and reproducing the optical disc. In order to detect these control signals by a compact arrangement at low cost, it is advantageous to employ a diffraction grating in the optical head device. In case recording and reproduction should be performed by the single optical information apparatus in a system including two or more light sources having different wavelengths, it is desirable that the diffraction element has an identical diffraction efficiency for the respective wavelengths of the light sources.
An arrangement in which a ratio of a zero-order beam (main beam) to first-order diffraction beams (sub-beams) can be adjusted for a specific wavelength is disclosed in Japanese Patent Laid-Open Publication Nos. 2001-281432, 2002-311219 and 2002-245660 and is described with reference to FIG. 12. FIG. 12 shows a conventional diffraction element 200. A first diffraction grating 200a for diffracting a laser beam of a wavelength λ1 is provided on one face of the conventional diffraction element 200, while a second diffraction grating 200b for diffracting a laser beam of a wavelength λ2 is provided on the other face of the conventional diffraction element 200. Thus, the first diffraction grating 200a diffracts the laser beam of the wavelength λ1 and transmits the laser beam of the wavelength λ2 therethrough as one beam. On the other hand, the second diffraction grating 200b diffracts the laser beam of the wavelength λ2 and transmits the laser beam of the wavelength λ1 therethrough as one beam. Meanwhile, a depth of the first diffraction grating 200a depends on the laser beam of the wavelength λ2 and a width of each of land portions and a width of each of groove portions of the first diffraction grating 200a are formed such that the ratio of the zero-order diffraction beam to the first-order diffraction beams of the laser beam of the wavelength λ1 diffracted by the first diffraction grating 200a falls within a predetermined range. Likewise, a depth of the second diffraction grating 200b depends on the laser beam of the wavelength λ1 and a width of each of land portions and a width of each of groove portions of the second diffraction grating 200b are formed such that the ratio of the zero-order diffraction beam to the first-order diffraction beams of the laser beam of the wavelength λ2 diffracted by the second diffraction grating 200a falls within a predetermined range.
In the above conventional arrangement, the diffraction gratings are, respectively, provided on opposite faces of a light-transmittable substrate, which requires time-consuming and expensive operations.
Meanwhile, since optical loss on the face of the diffraction grating, through which the beam is transmitted totally, is not zero, optical loss of the diffraction element having the two diffraction gratings provided on the opposite faces, respectively becomes large accordingly.
Meanwhile, in the above prior art documents, an infrared beam having a wavelength of 785 to 790 nm for the CD and a red beam having a wavelength of 650 to 658 nm for the DVD are used as the two wavelengths. In the next-generation ultrahigh-density optical disc apparatus, since a blue beam having a wavelength of 380 to 420 nm is used, an element usable for the blue beam should be provided. However, the above prior art documents do not disclose an arrangement including such an element.